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Abstract:

According to one embodiment, a nonaqueous electrolyte battery includes a
nonaqueous electrolyte, a positive electrode, a negative electrode and a
separator. The nonaqueous electrolyte includes an asymmetric
sulfone-based compound and a symmetric sulfone-based compound. The
positive electrode includes a composite oxide represented by
Li1-xMn1.5-yNi0.5-zMy+zO4 (0≦x≦1,
0≦y+z≦0.15, and M is at least one kind of element selected
from the group consisting of Mg, Al, Ti, Fe, Co, Ni, Cu, Zn, Ga, Nb, Sn,
Zr and Ta). The negative electrode includes a Ti-containing oxide which
is capable of absorbing and releasing lithium. The separator includes a
nonwoven fabric.

Claims:

1. A nonaqueous electrolyte battery comprising: a nonaqueous electrolyte
comprising a nonaqueous solvent containing an asymmetric sulfone-based
compound represented by the formula 1 and a symmetric sulfone-based
compound represented by the formula 2, and a lithium salt which is
dissolved in the nonaqueous solvent; a positive electrode comprising a
positive electrode active material containing a composite oxide
represented by Li1-xMn.sub.1.5-yNi.sub.0.5-zMy+zO4
(0.ltoreq.x≦1, 0.ltoreq.y+z≦0.15, and M is at least one
kind of element selected from the group consisting of Mg, Al, Ti, Fe, Co,
Ni, Cu, Zn, Ga, Nb, Sn, Zr and Ta); a negative electrode comprising a
negative electrode active material containing a Ti-containing oxide which
is capable of absorbing and releasing lithium; and a separator comprising
a nonwoven fabric which is provided between the positive electrode and
the negative electrode ##STR00007## wherein R.sub.1.noteq.R2, and
R1 and R2 are each an alkyl group having 1 to 10 carbon atoms,
and R3 is an alkyl group having 1 to 6 carbon atoms.

2. The battery according to claim 1, wherein the symmetric sulfone-based
compound has a melting point of 100.degree. C. or less.

3. The battery according to claim 1, wherein the symmetric sulfone-based
compound has a melting point of from 25 to 100.degree. C.

4. The battery according to claim 1, wherein the nonwoven fabric
comprises at least one of cellulose and a polyolefin.

5. The battery according to claim 1, wherein an amount by mole of the
symmetric sulfone-based compound is equal to or more than an amount by
mole of the lithium salt.

6. The battery according to claim 1, wherein the symmetric sulfone-based
compound is a solid at a room temperature, and the symmetric
sulfone-based compound is dissolved in the asymmetric sulfone-based
compound by an amount that is equal to or less than a saturation
dissolution amount at a room temperature.

7. The battery according to claim 1, wherein the lithium salt comprises
at least one of LiPF6 and LiBF.sub.4.

8. The battery according to claim 1, wherein R1 and R2 are each
a methyl group, an ethyl group, a butyl group or an isopropyl group, and
are different from each other.

9. The battery according to claim 1, wherein R3 is a methyl group,
an ethyl group, a butyl group or an isopropyl group.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is based upon and claims the benefit of priority
from Japanese Patent Application No. 2011-209232, filed Sep. 26, 2011,
the entire contents of which are incorporated herein by reference.

FIELD

[0002] Embodiments described herein relate generally to a nonaqueous
electrolyte battery and a battery pack.

BACKGROUND

[0003] Among secondary batteries, nonaqueous electrolyte secondary
batteries are secondary batteries that are charged and discharged by the
transfer of lithium ions between positive and negative electrodes, and
since the nonaqueous electrolyte secondary batteries use an organic
solvent as an liquid electrolyte, they can provide a larger voltage than
those provided by nickel-cadmium secondary batteries and nickel metal
hydride secondary batteries that use an aqueous solution. In nonaqueous
electrolyte secondary batteries that are practically used now,
lithium-containing cobalt composite oxides and lithium-containing nickel
composite oxides are used as positive electrode active materials, and
carbon-based materials, lithium titanate and the like are used as
negative electrode active materials. Furthermore, as a liquid
electrolyte, those obtained by dissolving a lithium salt such as
LiPF6 and LiBF4 in an organic solvent such as cyclic carbonates and
chain carbonates are used.

[0004] The positive electrode active material has an average working
potential of about from 3.4 to 3.8 V versus Li/Li+, and the maximum
potential during charging of from 4.1 to 4.3 V versus Li/Li+. V
versus Li/Li+is a unit of potential based on a metallic lithium
potential. On the other hand, the carbon-based material and lithium
titanate that are negative electrode active materials have average
working potentials of about from 0.05 to 0.5 V and 1.55 V, respectively,
versus Li/Li+. By combining these positive and negative electrode
active materials, the battery voltage becomes from 2.2 to 3.8 V, and the
maximum charge voltage becomes from 2.7 to 4.3 V.

[0005] As a countermeasure for further improving a capacity, use of
LiMn1.5Ni0.5O4 having the maximum potential during
charging of from 4.4 V to 5.0 V as a positive electrode active material
is suggested. However, there was a problem that, when this positive
electrode active material is used together with an electrolyte containing
a carbonate-based solvent, the solvent causes an oxidation reaction at
the positive electrode during charging, thereby the battery performance
is deteriorated and generation of gas is caused. On the other hand, use
of sultone- or sulfone-based compounds also as a solvent has been
suggested. However, there are problems that sultone- and sulfone-based
compounds have higher viscosity, provide lower solubility of lithium
salts and have higher reactivity with a negative electrode as compared to
carbonate-based solvents.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] FIG. 1 is a schematic view showing the cross-sectional surface of
the nonaqueous electrolyte battery according to the first embodiment;

[0007] FIG. 2 is an enlarged sectional view of the portion A in FIG. 1;

[0008] FIG. 3 is an exploded perspective view of the battery pack
according to the second embodiment;

[0009] FIG. 4 is a block diagram showing the electric circuit of the
battery pack of FIG. 3;

[0010] FIG. 5 is a schematic view of the cross-sectional surface of the
nonaqueous electrolyte battery of Examples;

[0011] FIG. 6 is a graph showing the discharge rate properties of the
nonaqueous electrolyte batteries of Examples and Comparative Example; and

[0012] FIG. 7 is a graph showing the discharge rate properties of the
nonaqueous electrolyte batteries of Examples and Comparative Example.

DETAILED DESCRIPTION

[0013] According to one embodiment, there is provided a nonaqueous
electrolyte battery including a nonaqueous electrolyte, a positive
electrode, a negative electrode and a separator. The nonaqueous
electrolyte includes a nonaqueous solvent and a lithium salt that is
dissolved in the nonaqueous solvent. The nonaqueous solvent contains an
asymmetric sulfone-based compound represented by the formula 1 and a
symmetric sulfone-based compound represented by the formula 2. The
positive electrode includes a positive electrode active material
containing a composite oxide represented by
Li1-xMn1.5-yNi0.5-zMy+zO4 (0≦x≦1,
0y+z≦0.15, and M is at least one kind of element selected from the
group consisting of Mg, Al, Ti, Fe, Co, Ni, Cu, Zn, Ga, Nb, Sn, Zr and
Ta). The negative electrode includes a negative electrode active material
containing a Ti-containing oxide which is capable of absorbing and
releasing lithium. The separator is provided between the positive
electrode and negative electrode, and includes a nonwoven fabric.

##STR00001##

[0014] wherein R1≠R2, and R1 and R2 are each an
alkyl group having 1 or more and 10 or less carbon atoms, and

##STR00002##

[0015] R3 is an alkyl group having 1 or more and 6 or less carbon
atoms.

[0016] Furthermore, according to one embodiment, a battery pack including
the nonaqueous electrolyte battery according to the embodiment is
provided.

[0017] The embodiments will be hereinafter explained with reference to
drawings.

First Embodiment

[0018] According to the first embodiment, a nonaqueous electrolyte battery
comprising a case, a positive electrode, a negative electrode, a
separator and a nonaqueous electrolyte is provided. The positive
electrode, negative electrode, separator and nonaqueous electrolyte are
housed in the case.

[0019] The nonaqueous electrolyte comprises a nonaqueous solvent and a
lithium salt that is dissolved in the nonaqueous solvent. The nonaqueous
solvent contains an asymmetric sulfone-based compound represented by the
formula 1 and a symmetric sulfone-based compound represented by the
formula 2.

##STR00003##

[0020] wherein R1≠R2, and R1 and R2 are each an
alkyl group having 1 or more and 10 or less carbon atoms, and

##STR00004##

[0021] R3 is an alkyl group having 1 or more and 6 or less carbon
atoms.

[0022] The positive electrode comprises a positive electrode active
material containing a composite oxide represented by
Li1-xMn1.5-yNi0.5-zMy+zO4 (0≦x≦1,
0≦y+z≦0.15, and M is at least one kind of element selected
from the group consisting of Mg, Al, Ti, Fe, Co, Ni, Cu, Zn, Ga, Nb, Sn,
Zr and Ta). The negative electrode comprises a negative electrode active
material containing a Ti-containing oxide which is capable of absorbing
and releasing lithium. The separator is disposed between the positive
electrode and negative electrode.

[0023] The oxide used for the positive and negative electrode active
materials is prepared by subjecting a plurality of kinds of carbonates
and hydroxides as raw materials to a calcination process. Therefore, even
raw materials in amounts that are required in principle are prepared,
remaining of the raw materials cannot be avoided due to a bad process for
mixing plural raw materials, an insufficient calcination period or the
like. Furthermore, since a specific raw material is used in a larger
amount than a stoichiometrically defined amount so as to increase a yield
in many cases, remaining of the raw materials occurs. Furthermore, since
the production and construction of electrodes are conducted in the air,
adsorption of moisture or CO2 is caused. Among these residual raw
materials and adsorbed substances, hydroxides, carbonates, water and the
like have a small electrochemical window, and thus they are decomposed
when a voltage is applied to positive and negative electrodes, thereby
gases such as hydrogen and carbon dioxide are generated. Since a 5 V
positive electrode comprising the above-mentioned composite oxide has the
maximum voltage during charging that is higher than that of a 4 V
positive electrode, oxidation of residual raw materials and adsorbed
substances occurs easily. Furthermore, since the above-mentioned oxide
used for the negative electrode is not a carbon-based material that is
calcined at a high temperature, remaining of the raw materials occurs
easily and generation of gas due to reduction occurs easily.

[0024] By using the electrolyte and separator as mentioned above in a
nonaqueous electrolyte battery equipped with such positive and negative
electrodes at which generation of gas easily occurs, swelling due to
generation of gas can be suppressed, and excellent output properties,
specifically excellent rate properties and cycle properties, and a large
charge capacity can be obtained. Since the compounds represented by the
formulas 1 and 2 are difficult to be oxidized, a nonaqueous electrolyte
battery having excellent output properties at a room temperature and a
high temperature of about 60° C. can be constituted by using these
compounds by mixing. Although a specific mechanism of improving
properties is unclear, it is presumed that the amount of the sulfone
group that is a polar group in a unit volume of the nonaqueous solvent
increases by mixing the symmetric sulfone-based compound represented by
the formula 2 which has a melting point at around a room temperature or
more, thereby the solubility of the lithium salt in the nonaqueous
solvent is increased. Furthermore, the symmetric sulfone-based compound
is more advantageous than the asymmetric sulfone-based compound since the
synthesis process therefor can be simplified.

[0025] Furthermore, simplification of the synthesis process is also
effective in view of improvement of the purity of the symmetric
sulfone-based compound.

[0027] The positive electrode contains a positive electrode active
material, and can also comprise a substance having electron conductivity
(hereinafter referred to as a conductive material) and a binder.
Furthermore, the positive electrode can comprise a current collector. The
current collector contacts with a positive electrode material layer
comprising the positive electrode active material. The positive electrode
material layer can be obtained by, for example, kneading the positive
electrode active material, conductive material and binder, and forming
the kneaded product into a sheet by pressing. Alternatively, it is also
possible to form the positive electrode material layer on the current
collector by preparing a slurry by dissolving or suspending the positive
electrode active material, conductive material and binder in a solvent
such as toluene and N-methylpyrrolidone (NMP), and applying the slurry to
the current collector and drying the slurry to form a sheet.

[0028] The positive electrode active material contains a composite oxide
represented by Li1-xMn1.5-yNi0.5-zMy+zO4
(0≦x≦1, 0≦y+z≦0.15, and M is at least one
kind of element selected from the group consisting of Mg, Al, Ti, Fe, Co,
Ni, Cu, Zn, Ga, Nb, Sn, Zr and Ta). The molar ratio of Li (1-x) may vary
in the range of 0≦x≦1 by the absorption and release of
lithium in accordance with a charge-discharge reaction. Although the
substitution amount (y+z) may be zero, 0.01 or more is desirable in view
of suppression of surface activity. Furthermore, it is desirable that the
substitution amount (y+z) is 0.15 or less from the viewpoint of
performance of high capacity. A more preferable range is 0.03 or more and
0.1 or less. Furthermore, by substituting a part of at least one of Mn
and Ni with the element M, the surface activity of the positive electrode
active material can be decreased, thereby increase in the battery
resistance can further be suppressed. Specifically, it is desirable to
use at least one of Mg and Zr as the element M since the substitution
effect is high. From the viewpoints of cycle property and costs, it is
desirable that the composite oxide is LiMn1.5Ni0.5O4. The
kind of the positive electrode active material as used can be one kind or
two or more kinds.

[0029] Examples of the conductive material can include carbon materials
and the like.

[0030] The binder includes at least one kind of polytetrafluoroethylene
(PTFE), polyvinylidene fluoride (PVdF), ethylene-propylene-diene
copolymers, styrene-butadiene rubbers or the like.

[0031] A base material having electron conductivity such as a metal can be
used for the current collector. Specific examples of the current
collector include metal foils, thin plates or meshes, metal meshes and
the like of aluminum, stainless, titanium and the like.

[0032] In the case when the positive electrode material layer contains the
positive electrode active material, conductive material and binder, the
mixing ratios of the positive electrode active material, conductive
material and binder are preferably 73 to 95% by weight of the positive
electrode active material, 3 to 20% by weight of the conductive material
and 2 to 7% by weight of the binder, with respect to 100% by weight of
the positive electrode material layer.

[0033] It is desirable that the positive electrode is used so that the
maximum potential during charging is in the range of from 4.4 to 4.9 V
(vs. Li/Li+). By adjusting the maximum potential during charging to
be within such range, a large capacity can be obtained. Specifically, it
is more desirable to use the positive electrode so that the maximum
potential during charging becomes the range of 4.6 to 4.8 V (vs.
Li/Li+) since a large capacity and suppression of side reactions can
be achieved simultaneously. Furthermore, it is desirable to use the
positive electrode so that the lowest potential during discharging
becomes the range of 4.0 to 4.3 V (vs. Li/Li+) since the
deterioration of the positive electrode active material can be minimized
and higher cycle properties can be obtained.

(Negative Electrode)

[0034] The negative electrode contains a negative electrode active
material, and can also comprise a conductive material and a binder.
Furthermore, the negative electrode can comprise a current collector. The
current collector is in contact with a negative electrode material layer
comprising the negative electrode active material. The negative electrode
material layer may be formed into a pellet form, a thin plate form or a
sheet form. The negative electrode material layer is obtained by kneading
the negative electrode active material, conductive material and binder,
and pressing the kneaded product to form a pellet or sheet.
Alternatively, the negative electrode material layer can be formed on the
current collector by kneading the negative electrode active material,
conductive material and binder, dissolving or suspending these in a
solvent such as water and N-methylpyrrolidone (NMP) to prepare a slurry,
and applying the slurry to the current collector and drying the slurry to
form a sheet.

[0035] Examples of the Ti-containing oxide which is capable of absorbing
and releasing lithium may include a spinel type lithium titanate, a
monoclinic system titanium dioxide and the like. The kind of the negative
electrode active material as used can be one kind or two or more kinds.
Since a spinel type lithium titanate provides high flatness to a
charge-discharge curve, when it is combined with a positive electrode
containing the above-mentioned composite oxide, the potentials of the
positive and negative electrodes can be controlled easily. Furthermore,
the spinel type lithium titanate can minimize potential variations due to
progression of cycles and realize high cycle properties.

[0036] A substance having electron conductivity such as carbon materials
and metals can be used for the conductive material. Forms of a powder, a
fibrous powder and the like are desirable.

[0037] As the binder, polytetrafluoroethylene (PTFE), polyvinylidene
fluoride (PVdF), styrene-butadiene rubber, carboxymethyl cellulose (CMC)
or the like can be used.

[0038] As the current collector, metal foils, thin plates or meshes, metal
meshes or the like can be used. Examples of the current collector
material may include copper, stainless, nickel, aluminum and the like.

[0039] In the case when the negative electrode material layer contains the
negative electrode active material, conductive material and binder, the
mix ratios of the negative electrode active material, conductive material
and binder are preferably adjusted to 73 to 96% by weight of the negative
electrode active material, 2 to 20% by weight of the conductive material
and 2 to 7% by weight of the binder, with respect to 100% by weight of
the negative electrode material layer.

[0040] Furthermore, it is desirable that the negative electrode is used so
that the lowest potential during charging becomes 1.0 V (vs. Li/Li+)
or more. By doing so, the side reactions between the negative electrode
and sulfone-based compounds can be suppressed. By adjusting the lowest
potential during charging to the range of 1.0 to 1.5 V (vs. Li/Li+),
side reactions in which the electrolyte in the cell is involved can be
minimized, thereby a high charge-discharge efficiency and cycle
performance can be realized. Specifically, it is more desirable to use
the negative electrode so that the lowest potential during charging
becomes the range of 1.35 to 1.45 V (vs. Li/Li+) since deterioration
of the negative electrode active material during cycles can further be
decreased and increase in impedance can be suppressed. Furthermore, it is
more desirable to use the negative electrode so that the maximum
potential during discharging becomes the range of 1.6 to 2.0 V (vs.
Li/Li+) since deterioration of the negative electrode active
material during cycles can further be decreased. Examples of the negative
electrode active material that can be operated under such potential may
include a spinel type lithium titanate, a monoclinic system titanium
dioxide and the like.

(Separator)

[0041] A nonwoven fabric separator is impregnated with the nonaqueous
electrolyte. Examples of the nonwoven fabric may include cellulose
nonwoven fabrics, polyethylene telephthalate nonwoven fabrics and
polyolefin nonwoven fabrics. A nonwoven fabric formed of at least one of
cellulose and a polyolefin is desirable since it has oxidation resistance
on the surface at which the positive electrode is contacted. However, a
film made of glass fibers which is generally classified into a glass
filter has a large glass fiber diameter and causes short-circuit unless
the film thickness is increased, and thus it can be used for a test in a
laboratory but cannot be used in an actual battery since the battery
capacity is impaired. In addition, there is also a problem that, when
glass is present on the surface of an electrode, the decomposition of
fluorine-containing anions is promoted and decrease in the properties is
caused.

[0042] For example, in porous films such as polyolefin porous films, pores
are formed by making cracks by stretching. Therefore, most of the pores
are formed in the direction vertical to the surface of the porous film.
The direction vertical to the surface of the porous film (the surface of
the separator) is a direction to which the positive and negative
electrodes are opposed and ions transfer. However, since gas generated on
the surface of the electrodes readily retains in pores that are in
parallel to this vertical direction, diffusion of the gas out of the
electrode group is disrupted. On the other hand, since a nonwoven fabric
is obtained by laminating and integrating fine fibers, voids are extended
in all directions, and there are paths through which gas goes in the
direction in parallel to the electrodes, i.e., paths through which gas
goes in the direction in parallel to the surface of the separator.
Therefore, generation of gas is suppressed by the nonaqueous electrolyte
and the generated gas diffuses quickly out of the electrode group, and
thus excellent rate properties and cycle performance, and a large charge
capacity can be obtained.

[0043] The thickness of the separator can be 8 μm or more and 40 μm
or less.

(Nonaqueous electrolyte)

[0044] The nonaqueous electrolyte comprises a nonaqueous solvent and a
lithium salt that is dissolved in the nonaqueous solvent. The nonaqueous
solvent contains an asymmetric sulfone-based compound represented by the
formula 1 and a symmetric sulfone-based compound represented by the
formula 2.

##STR00005##

[0045] wherein R12, and R1 and R2 are each an alkyl group
having 1 or more and 10 or less carbon atoms, and

##STR00006##

[0046] R3 is an alkyl group having 1 or more and 6 or less carbon atoms.

[0047] Examples of the alkyl group having 1 to 10 carbon atoms to be
R1 and R2 in the formula 1 may include a methyl group, an ethyl
group, a butyl group and an isopropyl group. For the asymmetric
sulfone-based compound, isopropyl methyl sulfone (abbreviation: IPMS) in
which R1 is a methyl group and R2 is an isopropyl group, ethyl
isopropyl sulfone (abbreviation: EIPS) in which R1 is an ethyl group
and R2 is an isopropyl group, and the like are desirable since they
have a low melting point and a low molecular weight. In addition,
n-butyl-n-propyl sulfone, ethyl-n-propyl sulfone and the like are
desirable since they have a low melting point.

[0048] Examples of the alkyl group having 1 to 6 carbon atoms to be
R3 in the formula 2 may include a methyl group, an ethyl group, a
butyl group, an isopropyl group and the like. It is desirable that the
symmetric sulfone-based compound has a smaller molecular weight than that
of the asymmetric sulfone-based compound since the rate of the sulfone
group that has polarity can be increased.

[0049] It is desirable that the symmetric sulfone-based compound has a
melting point of 100° C. or less since the mixed solvent can
retain a liquid state in a wide temperature range to prevent a
precipitate. In addition, if the melting point is 100° C. or less,
the symmetric sulfone-based compound can be formed into a liquid state by
heating in a water bath, which makes a process for improving purity and a
process for the preparation of the mixed solvent easy. The effect that
excellent rate properties and cycle properties and a large charge
capacity can be obtained becomes more significant at a high temperature
area at about 45 to 60° C. than at a room temperature. A more
preferable range of the melting point is the range of 25 to 100°
C. The melting point of the symmetric sulfone-based compound is about
108° C. when R3 is a methyl group, about 73° C. when
the group is an ethyl group, about 29.5° C. when the group is an
n-propyl group, and about 44° C. when the group is an n-butyl
group.

[0050] It is desirable that the molecular weight of the symmetric
sulfone-based compound is lower than the molecular weight of the
asymmetric sulfone-based compound, since a symmetric sulfone-based
compound that is more inexpensive and has a larger sulfone group content
rate in the molecule can be used. As mentioned above, by increasing the
rate of the sulfone group that has polarity against the alkyl group that
is nonpolar, the oxidation resistance of the nonaqueous solvent can
further be increased, and the output properties at a room temperature and
at a high temperature of about 60° C. can further be increased.
Furthermore, since the symmetric sulfone-based compound is more
inexpensive than the asymmetric sulfone-based compound due to its
symmetric property, the output properties can be improved at low costs by
adjusting the molecular weight as mentioned above.

[0051] It is desirable to mix the symmetric sulfone-based compound by an
amount by mole that is equal to or more than the amount by mole of the
lithium salt dissolved in the solvent so that the symmetric sulfone-based
compound can sufficiently contribute to the dissociation of the lithium
salt. On the other hand, since the symmetric sulfone-based compound that
has a lower molecular weight than that of the asymmetric sulfone-based
compound is a solid at a room temperature (25° C.), it is
desirable that the symmetric sulfone-based compound does not precipitate
or deposit in the mixed solvent. For the above-mentioned reasons, it is
desirable that the symmetric sulfone-based compound is dissolved in the
asymmetric sulfone-based compound in an amount that is equal to or less
than a saturation dissolution amount at a room temperature. However, if
no deposition is observed after dissolution of the lithium salt, the
symmetric sulfone-based compound can be used even if it is in an amount
that exceeds a saturation dissolution amount at a room temperature with
respect to the asymmetric sulfone-based compound. In this case, it is
possible that precipitation of the symmetric sulfone-based compound
occurs in the case when the lithium salt is decreased by the reaction in
the battery. In addition, in the case when the battery is mainly used at
a room temperature or more, the symmetric sulfone-based compound can be
used if it is in an amount that is equal to or less than a saturation
dissolution amount at an assumed temperature for use.

[0052] Each of the symmetric sulfone-based compound and asymmetric
sulfone-based compound is not necessarily limited to one kind, and two or
more kinds can be used for each.

[0053] In a way other than those mentioned above, it is also possible to
decrease the viscosity, improve the solubility of the lithium salt and
decrease the melting point by mixing various solvents. However, in order
to minimize the effect of the oxidative decomposition of the solvents
other than the sulfone-based compounds, it is desirable that the amount
of mixing is 10% by weight or less of the solvent. As the above-mentioned
various solvents, ethylene carbonate (EC), propylene carbonate (PC),
dimethyl carbonate (DMC), methyl ethyl carbonate (MEC), diethyl carbonate
(DEC), y-butyrolactone acetonitrile (AN), ethyl acetate (EA), toluene,
xylene or methyl acetate (MA), or the like can be used. The kind of the
solvent as used can be one kind or two or more kinds. In order to
compensate for the low solubility of the lithium salt in the
sulfone-based compounds, cyclic carbonates such as EC and PC that have a
high dielectric constant and in which the solubility of the lithium salt
is high are desirable. Furthermore, from the viewpoint of suppression of
generation of gas, cyclic carbonates such as EC and PC are more desirable
than chain carbonates such as DMC, DEC and MEC.

[0054] As the lithium salt, lithium perchlorate (LiCO4), lithium
hexafluorophosphate (LiPF6), lithium tetrafluoroarsenic
(LiBF4), lithium trifluoromethylsulfonate, lithium
bis(trifluoromethylsulfonyl)imide (LiTFSI), lithium
bispentafluoroethylsulfonylimide or the like can be used. LiPF6 or
LiBF4 is desirable since longer cycle properties can be obtained,
and a mixed salt thereof may also be used.

(Case)

[0055] Examples of the case include cans made of a metals or resin,
containers made of a laminate film, rectangular containers comprising a
plastic, a ceramic or the like, and the like. As the metal can, a
rectangular container of aluminum, iron, stainless or the like can be
used. A case made of a laminate film can be obtained, for example, by
forming a laminate film material comprising a metal layer of aluminum,
copper, stainless or the like and a resin layer into a saclike form by
heat sealing. The case made of a laminate film is specifically desirable
since generation of gas in the cell can be detected as a change in the
appearance of the battery.

[0056] An example of the nonaqueous electrolyte secondary battery
according to the first embodiment is shown in FIG. 1 and FIG. 2. FIG. 1
is a drawing that schematically shows a cross-sectional surface obtained
by cutting a flat type nonaqueous electrolyte secondary battery in the
direction of the thickness of the battery, and FIG. 2 is an enlarged
sectional view showing the portion A in FIG. 1. The nonaqueous
electrolyte secondary battery comprises a case 1 made of a laminate film,
an electrode group 2 housed in the case 1, and a nonaqueous electrolyte
(not depicted).

[0057] The case 1 made of a laminate film is obtained by forming a
laminate film comprising a metal layer and a resin layer into a saclike
form by heat sealing. The electrode group 2 is obtained by stacking sets
of a positive electrode 3 and a negative electrode 4 and a separator 5
interposed therebetween. The positive electrode 3 comprises a positive
electrode current collector 3a and positive electrode material layer(s)
3b that is/are formed at both sides or one side of the positive electrode
current collector 3a. The negative electrode 4 comprises a negative
electrode current collector 4a and negative electrode material layer(s)
4b that is/are formed at both sides or one side of the negative electrode
current collector 4a. A band-like positive electrode terminal 6 is
electrically connected to the positive electrode current collector 3a of
the positive electrode 3, and the tip thereof is extended to outside
through the heat sealed portion of the case 1. On the other hand, a
band-like negative electrode terminal 7 is electrically connected to the
negative electrode current collector 4a of the negative electrode 4, and
the tip thereof is extended to outside through the heat sealed portion of
the case 1.

[0058] The positive electrode terminal is electrically connected to the
positive electrode, and has a function to electrically bridge the outside
of the battery and the positive electrode. The form of the positive
electrode terminal is not limited to the band-like form as shown in FIG.
1, and can also be, for example, a ribbon-like shape or a rod-like shape.
Furthermore, a part of the positive electrode current collector may be
used or a member that is different from the positive electrode current
collector may be used for the positive electrode terminal. The positive
electrode terminal can be formed from, for example, aluminum, an aluminum
alloy, titanium or the like.

[0059] The negative electrode terminal is electrically connected to the
negative electrode, and has a function to electrically bridge the outside
of the battery and the negative electrode. The form of the negative
electrode terminal is not limited to the band-like form as shown in FIG.
1, and can also be, for example, a ribbon-like shape or a rod-like shape.
Furthermore, a part of the negative electrode current collector may be
used or a member that is different from the negative electrode current
collector may be used for the negative electrode terminal. The negative
electrode terminal can be formed from, for example, aluminum, an aluminum
alloy, copper, stainless or the like. Aluminum and aluminum alloys are
desirable since they have a light weight and excellent weld-connecting
property.

[0060] Although a nonaqueous electrolyte battery comprising a stacking
electrode group and a case made of a laminate film is shown in FIG. 1 and
FIG. 2, the shape of the electrode group and the kind of the case in the
nonaqueous electrolyte battery are not limited to those shown in the
drawings, and any shape and kind can be used as long as they can be used
for a nonaqueous electrolyte battery. For example, it is possible to use
a wound type electrode group or to use a metal can for the case.

[0061] According to the first embodiment, since the nonaqueous electrolyte
comprising the nonaqueous solvent containing the asymmetric sulfone-based
compound represented by the formula 1 and the symmetric sulfone-based
compound represented by the formula 2 and the nonwoven fabric separator
are used, the output properties such as rate properties of the nonaqueous
electrolyte battery comprising the positive electrode comprising the
composite oxide represented by
Li1-xMn1.5-yNi0.5-zMy+zO4 (0≦x≦1,
0y+z≦0.15, and M is at least one kind of element selected from the
group consisting of Mg, Al, Ti, Fe, Co, Ni, Cu, Zn, Ga, Nb,

[0062] Sn, Zr and Ta) and the negative electrode comprising the
Ti-containing oxide which is capable of absorbing and releasing lithium
can be improved. Since the positive electrode can adjust the maximum
potential during charging to a high potential of from 4.4 to 4.9 V (vs.
Li/Li+), it is possible to realize a 3 V nonaqueous electrolyte
battery by combining the positive electrode with the negative electrode.
Therefore, according to the first embodiment, the output properties of
the 3 V nonaqueous electrolyte battery can be improved.

Second Embodiment

[0063] The battery pack according to the second embodiment has one or a
plurality of nonaqueous electrolyte secondary battery (batteries) (unit
cell(s)) of the first embodiment. When a plurality of unit cells are
included, the unit cells are electrically connected in series or in
parallel.

[0064] Such battery pack will be explained in detail with referring to
FIG. 3 and FIG. 4.

[0065] As the unit cell, for example, a flat type nonaqueous electrolyte
secondary battery can be used.

[0066] A plurality of unit cells 21 constituted by flat type nonaqueous
electrolyte secondary batteries are laminated so that a positive
electrode terminal 16 and a negative electrode terminal 17 that are
extending outward are aligned in the same direction, and are bound by an
adhesive tape 22 to constitute a battery module 23. As shown in FIG. 4,
the unit cells 21 are electrically connected in series with one another.

[0067] A printed circuit board 24 is disposed opposing to the side surface
of the unit cells 21 from which the negative electrode terminal 17 and
positive electrode terminal 16 are extended. As shown in FIG. 4, a
thermistor 25, a protective circuit 26, and a terminal 27 for carrying a
current to an external device are mounted on the printed circuit board
24. In addition, an insulating board (not shown) is attached to the
surface of the protective circuit substrate 24, which faces the battery
module 23, so as to avoid unnecessary connection with the conductors of
the battery module 23.

[0068] A positive electrode lead 28 is connected to the positive electrode
terminal 16 that is positioned at the lowermost layer of the battery
module 23, and the tip thereof is inserted to and electrically connected
to a positive electrode connector 29 of the printed circuit board 24. A
negative electrode lead 30 is connected to the negative electrode
terminal 17 that is positioned at the uppermost layer of the battery
module 23, and the tip thereof is inserted to and electrically connected
to a negative electrode connector 31 of the printed circuit board 24.
These connectors 29 and 31 are connected to a protective circuit 26 via
wirings 32 and 33 that are formed on the printed circuit board 24.

[0069] The thermistor 25 detects the temperature of the unit cells 21, and
the detection signal thereof is sent to the protective circuit 26. The
protective circuit 26 may break a positive conductor 34a and a negative
conductor 34b between the protective circuit 26 and the terminal 27 for
carrying a current to an external device, under a predetermined
condition. The predetermined condition refers to, for example, the time
at which the detection temperature of the thermistor 25 reaches a
predetermined temperature or more. Furthermore, the predetermined
condition refers to the time at which over-charge, over-discharge,
over-current or the like of the unit cells 21 are detected. The detection
of over-charge or the like is performed in the individual unit cells 21
or the battery module 23. When detection is performed in the individual
unit cell 21, a battery voltage may be detected, or a positive electrode
potential or negative electrode potential may be detected. In the latter
case, a lithium electrode that is used as a reference electrode is
inserted in the individual unit cell 21. In the case of FIGS. 3 and 4,
conductors 35 for detection of a voltage are connected to the respective
unit cells 21, and detection signals are sent to the protective circuit
26 via the conductors 35.

[0070] Protective sheets 36 made of a rubber or resin are disposed
respectively on the three side surfaces of the battery module 23 except
for the side surface from which the positive electrode terminal 16 and
negative electrode terminal 17 protrude.

[0071] The battery module 23 is housed in a housing container 37 together
with the respective protective sheets 36 and the printed circuit board
24. Namely, the protective sheets 36 are disposed respectively on the
both inner surfaces in the longitudinal side direction and the inner
surface in the short side direction of the housing container 37, and the
printed circuit board 24 is disposed on the inner surface on the opposite
side in the short side direction. The battery module 23 is positioned in
a space surrounded by the protective sheets 36 and the printed circuit
board 24. A lid 38 is attached to the upper surface of the housing
container 37.

[0072] Alternatively, the battery module 23 may be fixed by using a heat
shrink tape instead of the adhesive tape 22. In this case, the protective
sheets are disposed on both side surfaces of the battery module, the
battery module is wound around a heat shrink tube, and the heat shrink
tube is shrank by heating to bind the battery module.

[0073] Although an embodiment in which the unit cells 21 are connected
with each other in series is shown in FIGS. 3 and 4, the unit cells may
be connected with each other in parallel so as to increase a battery
capacity. Alternatively, assembled battery packs may be connected with
each other in series or parallel.

[0074] Furthermore, the embodiment of the battery pack is suitably changed
according to use. Preferable use of the battery pack is one for which
cycle performance at high rate is desired. Specific examples may include
uses in power sources for digital cameras, and in-car uses in two to
four-wheeled hybrid battery automobiles, two to four-wheeled battery
automobiles, motor assisted bicycles and the like. In-car uses are
preferable.

[0075] According to the second embodiment, since the battery pack
comprises the nonaqueous electrolyte battery according to the first
embodiment, a battery pack using a 3 V nonaqueous electrolyte battery
having improved output properties such as rate properties can be
realized.

EXAMPLES

[0076] Hereinafter Examples will be explained in detail by using the
drawings and tables. First, the nonaqueous electrolyte battery used in
Examples will be explained with referring to FIG. 5.

[0077] As shown in FIG. 5, the nonaqueous electrolyte battery of Examples
uses a glass container 41 as a case and a glass lid 42 disposed on the
opening of the glass container 41. The electrode group is a laminate in
which a positive electrode 43, a negative electrode 44, and a separator
45 disposed between the positive electrode 43 and negative electrode 44
are laminated. A positive electrode current collector plate 46 made of
titanium is laminated on the positive electrode 43. A titanium wire 47 is
connected to the positive electrode current collector plate 46. A
negative electrode current collector plate 48 made of nickel is laminated
on the negative electrode 44. A nickel wire 49 is connected to the
negative electrode current collector plate 48. Resin presser plates 50
are disposed on the both outermost layers of the electrode group, and the
electrode group is interposed between the resin presser plates 50. The
tips of the titanium wire 47 and nickel wire 49 are extended to outside
through the glass lid 42. A nonaqueous electrolyte 51 is housed in the
glass container 41, and the electrode group is immersed in the nonaqueous
electrolyte 51.

Example 1

[0078] 90% by weight of a powder of LiMn1.5Ni0.5O4 as a
positive electrode active material, 2% by weight of acetylene black, 5%
by weight of graphite, and 5% by weight of polyvinylidene fluoride as a
binder were formed into a slurry by using N-methylpyrrolidone as a
solvent. The slurry was applied to the both surfaces of an aluminum foil
having a thickness of 15 μm, dried and pressed. Thereafter one surface
was peeled off to expose the aluminum foil, and the aluminum foil was cut
into a 20 mm square to prepare a positive electrode sheet 43 of 20 mm
square.

[0079] 90% by weight of a powder of Li4Ti5O12 as a negative
electrode active material, 5% by weight of graphite as a conductive
material, and 5% by weight of polyvinylidene fluoride (PVdF) were added
to a solution of N-methylpyrrolidone (NMP) and mixed to prepare a slurry,
and the obtained slurry was applied to the both surfaces of an aluminum
foil having a thickness of 25 μm, dried and pressed. Thereafter one
surface was peeled off to expose the aluminum foil, and the aluminum foil
was cut into a 20 mm square to prepare a negative electrode sheet 44 of
20 mm square.

[0080] As a separator 45, a cellulose nonwoven fabric of 25 mm square
having a thickness of 20 μm was used.

[0081] A positive electrode current collector 46 to which a titanium wire
47 had been connected, the positive electrode 43, the separator 45, the
negative electrode 44, and a negative electrode current collector 48 to
which a nickel wire 49 had been connected were laminated in this order,
and the upper and bottom surfaces of the obtained laminate were pressed
by presser plates 50 to prepare an electrode group. The electrode group
was housed in a glass container 41 in an argon glovebox.

[0082] Ethyl isopropyl sulfone (abbreviation: SIPS) that is an asymmetric
sulfone-based compound having a melting point of -6° C. and
dipropyl sulfone (abbreviation: DPS) that is a symmetric sulfone-based
compound having a melting point of 29.5° C. that is a room
temperature or more were mixed at a weight ratio of 1:1. This mixed
solution was a liquid at a room temperature. Furthermore, the symmetric
sulfone-based compound was dissolved in the asymmetric sulfone-based
compound in an amount that is equal to or less than a saturation
dissolution amount at a room temperature. A nonaqueous electrolyte 51 was
prepared by dissolving 1 M of LiBF4 in the mixed solution. The molar
ratio of the lithium salt (LiBF4), EIPS and DPS was 1:3.8:3.3. The
nonaqueous electrolyte 51 was poured into the glass container 41, and the
glass container 41 was sealed with the glass lid 42 to give a nonaqueous
electrolyte battery shown in FIG. 5.

Example 2

[0083] Ethyl isopropyl sulfone (abbreviation: EIPS) that is an asymmetric
sulfone-based compound and dibutyl sulfone (abbreviation: DBS) that is a
symmetric sulfone-based compound having a melting point of 44° C.
that is not less than a room temperature were mixed at a weight ratio of
EIPS:DBS=1:1. This mixed solution was a liquid at a room temperature.
Furthermore, the symmetric sulfone-based compound was dissolved in the
asymmetric sulfone-based compound in an amount that is equal to or less
than a saturation dissolution amount at a room temperature. A nonaqueous
electrolyte was prepared in a similar manner to Example 1, except that
the obtained mixed solution was used. The molar ratio of the lithium salt
(LiBF4), EIPS and DBS was 1:3.8:2.7. A nonaqueous electrolyte
battery was prepared in a similar manner to Example 1, except that the
obtained nonaqueous electrolyte was used.

Comparative Example 1

[0084] A nonaqueous electrolyte battery was prepared in a similar manner
to Example 1, except that a polypropylene porous film having a thickness
of 12 μm was used as a separator.

[0085] The obtained nonaqueous electrolyte batteries of Examples and
Comparative Example were each charged under a 25° C. circumstance
at a constant current of 0.32 mA and a constant voltage of 3.25 V up to
15 hours, and discharged at 0.32 mA up to 2.7 V.

[0086] Thereafter the charging was stopped at the time when the charging
current reached 0.08 mA at a constant current of 0.64 mA and a constant
voltage of 3.25 V. The battery was discharged at 0.32 mA up to 2.7 V.
Thereafter the battery was charged in a similar manner, and discharging
was conducted at 0.64 mA, 1.6 mA and 3.2 mA.

[0087] Thereafter the battery was charged in a similar manner at
25° C., and a similar test in which the discharging current was
changed was conducted under a 0° C. circumstance.

[0088] It is understood from FIG. 6 that shows the results of the
above-mentioned evaluations at a room temperature (25° C.) that
higher rate properties can be obtained in Example 1 as compared to
Comparative Example 1. It is understood that properties that are equal to
or higher than those in Comparative Example 1 in which DPS was used could
be obtained even in the case when DBS having a higher viscosity was used
(Example 2). It is expected that poorer properties than those in
Comparative Example 1 are obtained in the case when a similar separator
to that in Comparative Example 1 is used and DBS is used, and thus it can
be said that high properties could be obtained as in the case of DPS by
the combination of the nonwoven fabric separator and the mixed solvent of
the sulfone compounds also in the case when DBS was used as the symmetric
sulfone.

[0089] Furthermore, as shown in FIG. 7, it was found that the difference
became significant at a low temperature (0° C.), Example 1 showed
significantly higher properties than those in Comparative Example 1, and
Example 2 in which DBS was used could provide an approximately similar
result to that in Comparative Example 1 in which DPS was used.
Furthermore, isopropyl methyl sulfone (abbreviation: IPMS) has a
structure that is extremely close to that of EIPS and can provide an
approximately similar level of ionic conductivity. Therefore, an
approximately similar result to that in Example in which EIPS was used
can be obtained even in the case when isopropyl methyl sulfone
(abbreviation: IPMS) is used instead of LIPS and a symmetric
sulfone-based compound such as DPS and DBS is dissolved in IPMS.
Specifically, it is considered in view of the temperature dependency of
ionic conductivity that a higher effect than that of EIPS can be obtained
at a room temperature or more.

[0090] According to the nonaqueous electrolyte battery of at least one
embodiment mentioned above, since the nonaqueous electrolyte comprising
the nonaqueous solvent containing the asymmetric sulfone-based compound
represented by the formula 1 and the symmetric sulfone-based compound
represented by the formula 2 and the nonwoven fabric separator are used,
the rate properties can be improved.

[0091] While certain embodiments have been described, these embodiments
have been presented by way of example only, and are not intended to limit
the scope of the inventions. Indeed, the novel embodiments described
herein may be embodied in a variety of other forms; furthermore, various
omissions, substitutions and changes in the form of the embodiments
described herein may be made without departing from the spirit of the
inventions. The accompanying claims and their equivalents are intended to
cover such forms or modifications as would fall within the scope and
spirit of the inventions.